1. Field of the Invention
The present invention relates generally to the isolation of radio frequency (RF) signal amplifiers from returned signals using circulator-based isolators.
2. Background and Related Art
Circulators are generally ferrite devices composed of permanent magnets. Circulators used as isolators pass RF signals and block returned signals. Some of the power in the RF signal, and nearly all of the power in the blocked returned signals, is dissipated as heat. Dissipating power as heat raises the temperature of a circulator. The performance characteristics of ferrite devices composed of permanent magnets change with temperature. In particular, the frequency response of ferrite, and the coercive effect of permanent magnets, changes as temperatures rise. Uncompensated changes in the frequency response of ferrite and the coercive effect of permanent magnets cause a circulator to suffer higher return losses, or "drift", in all of its ports. Drift manifests itself as a change in impedance. Changed impedances cause mis-matched impedances, which cause power to be reflected rather than transferred, which leads to further heating, further losses, and ultimately failure of the circulator. Failure of a circulator means upstream components such as amplifiers are no longer being isolated from returned signals, which jeopardizes their lives as well.
In general, the higher the temperature a circulator is able to withstand, the higher its rated power level. Circulators used as isolators are normally temperature-compensated to increase the temperature they are able to tolerate. Temperature compensation is costly. Higher operating temperatures, and hence higher rated power levels, may be achieved in return for higher cost and greater complexity. Still, there is a finite limit to the amount of power that can be passed safely through any circulator-based isolator.